In rabbits previously injected i.v. with alloxan, serial observations of platelet thrombus formation in response to topical adenosine diphosphate (ADP) at sites of electrical injuries in pial arteries have been made. Using this model we have studied the effects of oral daily doses of dipyridamole (Persantin) and acetyl salicylic acid (ASA) upon platelet thrombus formation. Oral daily doses of 42 mg of ASA and 6 mg dipyridamole given separately in alloxan-treated rabbits are without effect. When given together orally, 42 mg and 6 mg respectively reduced the level of sensitivity to ADP for producing platelet thrombi to that established for the rabbits before the injection of alloxan. But withdrawal of these combined doses of dipyridamole and ASA caused the sensitivity of ADP for platelet thrombus formation to be raised to the much increased level present in rabbits soon after they are given i.v. alloxan. This apparent synergistic behaviour displayed by dipyridamole and ASA in these rabbits results in antithrombotic effects which are clearly absent when these two agents are given separately. It is of interest that the dose levels used here are equivalent, an a body weight ratio, to those being used in man in the current Persantin-Aspirin Reinfarction Study.